Sunscreen compositions are frequently used to protect skin that is exposed to the sun for a variety of reasons such as sun bathing, or spending leisure time or working out of doors. Topical sunscreen compositions, which are easily applied to the skin, are usually in the form of a lotion, oil, cream or emulsion (water-in-oil and oil-in-water). Sunscreen compositions contain sunscreen agents to protect the skin from the harmful UV rays of the sun. These rays are generally in the form of UV-A and UV-B radiation which range from about 290 to 400 nm in wavelength.
There are short and long term hazards associated with prolonged exposure to UV radiation. Some of the long term effects include malignant changes in the skin surface, premature aging of the skin as evidenced by wrinkles, yellowing, cracking, telangiectasis (spider vessels), solar keratoses (growths), ecchymoses (subcutaneous hemorrhagic lesions), and loss of elasticity (sagging). A major short term effect of prolonged exposure to UV light is erythema, commonly known as a sunburn.
The amount of photoprotection against erythema is the basis for the determination of the SPF ("sun protection factor") value. The SPF value measures the amount of protection from the sun provided before a certain level of erythema is experienced. Compositions having higher SPF values are preferred because they offer more protection against the harmful effects caused by the sun and UV radiation.
Sunscreen agents act by absorbing, scattering or blocking UV radiation and thus, prevent UV radiation from penetrating the skin. They are available as both organic and inorganic agents. Typical organic agents include. for example. PABAs (p-aminobenzoic acids), benzophenones. salicylate esters and dioxybenzone. Examples of inorganic agents include zinc oxide, titanium dioxide and calamine. To achieve higher SPF values, typically, greater quantities of the sunscreen agent or combination of sunscreen agents are added to the composition. However, greater quantities of sunscreen agents present certain challenges in formulation, especially with respect to stability. For example, titanium dioxide tends to agglomerate and become less effective as a sunscreen agent. It is also a frequent complaint that sunscreens containing particularly high concentrations of titanium dioxide have an unpleasant or grimy feel on the skin and result in a white or blue hue on the skin. Other negative qualities that result from using high concentrations of inorganic sunscreen agents are the opaqueness of the formula when a clear formula is desired. the change in color of the formula, or other adverse aesthetic effects.
Producing a topical sunscreen composition with a high SPF is difficult to achieve without the negative characteristics associated with using larger quantities of sunscreen agents. Efforts to "boost" the SPF value of a sunscreen composition are demonstrated in U.S. Pat. Nos. 5,468,471 and 5,573,754. However, they include components in addition to the sunscreen agent that are relatively costly and not easily manufactured. Thus, there is a continued effort to find ways of boosting the SPF of topical sunscreen compositions. It is, therefore, an object of the present invention to reduce the irritation potential of topical sunscreen compositions to levels which will be acceptable to the average user of the product and to provide a topical sunscreen composition that is appealing to the consumer.